Such mass flow meters have long been known. The measuring principle of thermal mass flow meters is based on the cooling of a heating element mounted on a holder when immersed into a flowing fluid. The flow which flows over the surface of the heating element absorbs heat from the latter and thus cools the heating element. The construction and behavior are illustrated in principle in FIG. 1. In this case, the quantity of heat absorbed by the flow depends on the temperature difference between the surface and the fluid, and on the flow itself. It can be described by a function{dot over (q)}=α(TO−TF),where{dot over (q)} is the quantity of heat dissipated,(TO−TF) is the temperature difference, andα is a constant of proportionality.The constant of proportionality α is in this case directly dependent on the flow and is a function of the mass flow density over the heating element α=ƒ(ρν)˜√{square root over (ρν)}. Now, if the temperature difference between the surface and the fluid, and also the heating power required to generate this temperature difference, are known, the mass flow over the heating element can thus be determined from this.
Thus, for practical application of such a thermal mass flow measurement, two temperature sensors, one of which is heated and used for the flow measurement, are now put into the flow as illustrated in FIG. 2. The second temperature sensor serves to measure the fluid temperature TF.
In general, the measurement is in this case carried out only statically with a constant heating power or a constant temperature difference between the heater and the flow. However, a pulsed mode of operation, which is evaluated with slightly more effort, could also be carried out in this case.
However, for all these measurements here it is important that a very accurate measurement of the heating power and the temperature difference is carried out. The quantity of heat given off to the flow cannot be measured directly in this case but is rather determined by measuring the electrical heating power used. However, due to the construction, the electrical heating power introduced is not completely given off to the flow directly from the sensor head but a part of the heat flows into the holder of the sensor head and from there it is given off to the surroundings or to the flow at a greater distance from the measuring element. Since this heat flux is included in the measurement of the mass flow, it directly influences the measured result and presents a great source of error when using a thermal mass flow meter. It is partially taken into consideration during the calibration of the mass flow meter. However, since it is very variable, depending in particular on the flow and temperature conditions in the flow, it can be considered only to a limited extent during calibration and thus still presents a great source of error. It is thus attempted to keep this heat-loss flux as low as possible during the development of a thermal mass flow meter in order to achieve a flow measurement that is as accurate as possible.
In order to reduce this influence, it is generally attempted to set the ratio of the direct heat flux into the flow and the losses into the holder to be as great as possible during the development of a thermal mass flow meter. That is to say, a very good thermal contact between the heater and the flow is created and, at the same time, the heat outflow into the holder is reduced by appropriate insulation. A possible embodiment is presented in U.S. Pat. No. 5,880,365. In general, the insulation in this case comprises the entire holder of the sensor head in order to create the best insulation possible.
The quantity of heat given off to the flow is a measure of the flow. If the correlation of quantity of heat given off to the flow directly via the sensor and indirectly via the holder is constant, a unique relationship between the quantity of heat and the mass flow can be determined by calibration. In this case, it is assumed that the heat-loss flux given off by the sensor and holder only depends on the flow and the construction of the sensor itself.
However, in real process conditions, the assumption of a constant correlation between quantities of heat given off directly and indirectly to the flow, depending only on the flow, proves to be false. The main cause of this error is contamination of the flow medium, which is deposited on the surfaces of the sensor and holder and thus leads to a change in the heat transfers. By way of example, if the heat transfer between the sensor and the gas deteriorates, the supplied quantity of heat dissipated via the holder is increased. The correlation between quantities of heat given off by the sensor and the holder found during calibration is changed due to the contamination.
The contamination is thus an unwanted effect which falsifies the calibration data and hence the measurement. Since the contamination of the sensor head cannot be detected, this results in dangerous falsified measured values.
The contaminated mass meter will show a measured value that is too low due to the heat losses into the holder. There is the danger of overfilling in filling processes. In the case of billing, a value that is too low is calculated.
Although the problem can be countered by frequent recalibrations, which are often complex, the sensor must be removed for these. Disadvantageously, the process has to be suspended for this purpose. Since the time of de-calibration due to contamination cannot be anticipated, calibration must be performed, often unnecessarily, at short time intervals. Calibration “on demand” is not possible.
In addition, in situ monitoring with a second, independent sensor is known. However, since both sensors are subject to the same contamination, parallel drift occurs, so that once again the error cannot be detected. Furthermore, installation and service costs are considerably increased by redundant sensor systems.